Since I’ve changed the internet cable connection from IPv6 to IPv4 and after the disconnection/restart from my router I can’t login with root at my DietPi anymore.
When I enter the password, it says “access denied”. The password was saved in Solar-Putty and wasn’t changed by me.
Nevertheless I can login as DietPi with my set password!?
Update:
I’ve changed the password now in Webmin and now I can login again with Solar-Putty.
But should I be worried? Was I hacked!? And does the authenfication error belongs to my IPv4 change? 
Not sure what happened but you should be using a private and public key pair to login anyway. It’s much more secure and avoids ‘wrong password’ problems like this.
Yes, actually that was probably what I did (but somehow wrong).
I generated keys, but had not really a clue, for what it is exactly for… it was very late and I tried some stuff… on the next day I didn’t think about my generated keys in the first place. The whole IPv4/IPv6 change has probably absolutely nothing to do with my authenfication problem I have noticed on the next day… yeah, stupid me, now I know better.
I am now using a key to log in with Putty on my server.